We are investigating the origin and significance of drastic changes in the histone variant composition of nucleosomes during development, proliferation and differentiation of different cell types of the mouse, by studying: a) The molecular interactions of nonallelic primary structure variants and minor species of histones to form high affinity histone complexes and nucleosomes, b) the distribution of the various histone components in chromosome regions in different functional states, c) the molecular mechanisms of histone incorporation into chromatin in absence of DNA replication, in normal, regenerating and neoplastic liver cells as well as erythroleukemia cells before and after induction of terminal differentiation.